Power supply apparatus for a capacitive load

ABSTRACT

The invention provides a power supply apparatus for supplying electric power to a capacitive load. The apparatus has a transformer, a positive half-period driver and a negative half-period driver supplying positive and negative half-periods of voltage to the first coil. The second coil forms an electric resonance circuit and supplies electric voltage to the load. Zero crossings of the voltage supplied to the first coil are determined from a third coil on the transformer, and alternation between positive and negative half-periods of voltage supplied to the first coil is done at the zero crossings of the voltage supplied to the first coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit and priorityto U.S. patent application Ser. No. 12/520,495, filed on Dec. 2, 2009,which is a U.S. National Phase application of PCT InternationalApplication Number PCT/EP2007/064053, filed on Dec. 17, 2007,designating the United States of America and published in the Englishlanguage, which is an International Application of and claims thebenefit of priority to U.S. Provisional Application No. 60/876,050,filed on Dec. 20, 2006. The disclosures of the above-referencedapplications are hereby expressly incorporated by reference in theirentireties

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power supply apparatuses for supplyingelectric power to a capacitive load. The invention also relates to amethod for operating such power supply apparatuses having a capacitiveload such as an ozone generating device coupled to the power supplyapparatus. Furthermore the invention relates to a high voltagetransformer suitable for use in such power supply apparatuses.

2. Description of the Related Art

An example of capacitive loads is an ozone generating device coupled toa power supply apparatus generating an AC voltage to be supplied to theozone generating device. Such power supply apparatuses have an inductiveoutput impedance, and when the ozone generating device is connected tothe output of the power supply apparatus, the inductive output impedanceof the power supply apparatus and the capacitive impedance of the ozonegenerating device form a resonance circuit having a resonance frequency.Such ozone generating devices are driven at frequencies and voltagesthat are sufficiently high to produce a corona discharge in the ozonegenerating device. Air containing oxygen (O₂) such as atmospheric air orpure oxygen is supplied to the ozone generating device, the coronaconverts oxygen molecules (O₂) in the ozone generating device to ozone(O₃), and air with an enhanced content of ozone in comparison to the airsupplied to the ozone generating device is supplied from the ozonegenerating device. The amount of ozone produced by the ozone generatingdevice increases with the voltage supplied to it, and for minimizinglosses in the supply apparatus driving the ozone generating device thepower supply apparatus should be operated at or near the resonancefrequency. In practice, however, for several reasons the resonancefrequency may not be constant and may vary over time and as a functionof operating parameters including temperature and pressure in thesupplied air/oxygen; exchanging the ozone generating device or partsthereof, e.g. for service or maintenance, may change the resonancefrequency due to differences or tolerances in capacitance; and theresonance frequency may also change with the voltage at which the ozonegenerating device is operated since the corona is a non-linearphenomenon. It would therefore be advantageous to have a power supplyapparatus which operates at the actual resonance frequency of theresonance circuit and which adapts its frequency of operation to theactual resonance frequency of the resonance circuit.

Ozone generating devices may be operated at voltage levels in the rangeof several kV, at frequencies of several kHz and at power levels ofseveral kW. The power supply apparatus may have a high voltagetransformer with a high voltage second coil as its output. Whendesigning high voltage and high frequency transformers specialconsiderations should be paid to the design of in particular the highvoltage coil to avoid arcing between windings of the high voltage coiland between the windings and other objects near the coils. Arcing itselfmay damage the high voltage coil and other components, but arcing willcreate ozone which may have undesired effects on the equipment and theenvironment. It would therefore be advantageous to have a high voltagetransformer with a high voltage coil where arcing between windings ofthe high voltage coil is reduced or even avoided.

On a commercial and industrial scale ozone is produced from oxygen, O₂,in a gas containing oxygen. The oxygen-containing gas can be atmosphericair or oxygen-enriched gas. Methods exist for extracting oxygen fromatmospheric air to produce oxygen-enriched gas. Ozone can be producedfrom oxygen mainly by two methods, one comprising irradiating the oxygenwith ultra violet light, the other comprising a corona discharge device.Providing oxygen-enriched gas and producing ozone from oxygen areprocesses that consume energy and the consumptions of energy and otherresources of the two processes are comparable.

In some applications where ozone is used a predetermined yield of ozoneis needed or prescribed, or the required yield of ozone may change. Asimple and straightforward way of adjusting the yield is to adjust onlythe electric power of the ozone-generating apparatus and leaving theflow or supply of oxygen-containing gas constant, or vice versa. This isnot optimized for minimizing the consumption of resources comprisingoxygen-containing gas and power supplied from the power supplyapparatus, and the desired yield may possibly not result or may even beimpossible to obtain.

OBJECT OF THE INVENTION

It is an object of the invention to provide a power supply apparatushaving an inductive output impedance for supplying electric power to acapacitive load where it is ensured that the resonance circuit formed bythe inductive output impedance and the capacitive load impedance isoperated at the resonance frequency and.

It is also an object of the invention to provide a method of operatingan ozone generating apparatus in order to minimize the consumption ofresources comprising oxygen-containing gas and power supplied from thepower supply apparatus.

It is a further object of the present invention to provide a highvoltage transformer with reduced risk of arcing between windings of thehigh voltage coil and which is suitable for handling voltages in the kVrange, frequencies in the kHz range and power levels in the kW range.

SUMMARY OF THE INVENTION

The invention provides a power supply apparatus for supplying electricpower to a capacitive load having a capacitive load impedance. Theapparatus comprises

-   -   a transformer with a first coil and a second coil,    -   a positive half-period driver and a negative half-period driver        arranged to alternatingly supply positive half-periods of        voltage and negative half-periods of voltage, respectively, to        the first coil,    -   the second coil is connectable to the capacitive load so as to        form an electric resonance circuit having a resonance frequency,        and to supply electric voltage to the load, and    -   a device for determining zero crossings of the voltage supplied        to the first coil and for causing alternation between positive        and negative half-periods of voltage supplied to the first coil        at the zero crossings of the voltage supplied to the first coil,        wherein the device for determining zero crossings comprises a        third coil on the transformer.

An effect of this is that alternation between positive and negativehalf-periods of voltage supplied to the first coil is controlled by theactual resonance frequency of the resonance circuit formed by the secondcoil of the transformer and the capacitive load.

Another effect is that electric switching noise from the switchingelements is avoided since switching is done at times with no or very lowvoltage across the switching elements.

Such a power supply apparatus is useful for supplying electric power toa capacitive load having a capacitive load impedance such as an ozonegenerating device, and in particular an ozone generating device in whicha suitable combination of frequencies and voltages that are sufficientlyhigh to produce a corona discharge in the ozone generating device.

Other examples of capacitive loads include, without limiting theinvention thereto:

-   -   reactors for the destruction or disintegration of substances or        gases. Examples of gases that are considered to have a negative        effect on the environment if released are Halon 1301 and other        gases having fire extinguishing properties, SF₆ and other gases        used e.g. for their electrical properties, and gases used in        cooling apparatuses;    -   piezo-electric transducers used e.g. for generating ultrasound        in a medium for cleaning of objects immersed into the medium;    -   electro-luminescent devices such as electro-luminescent films        for use in LCD screens and in signs; and    -   devices for producing light arcs or corona discharges. Such        devices are used egg for producing ozone from an        oxygen-containing gas.

The device for determining zero crossings may sense the voltage itself,but in high voltage applications this may not be feasible, and thedevice may then comprise a separate coil on the transformer. Thisensures that the sensed voltage is in phase with the voltages in thecoils, whereby it is ensured that alternating between positive andnegative half-periods of voltage supplied to the first coil is actuallydone at the zero crossings of the voltage supplied to the first coil.

In an embodiment each of the positive and negative half-period driversis arranged to feed a voltage through an inductive element to the firstcoil for a duration of no more than one quarter of a periodcorresponding to a predetermined highest resonance frequency. Theinductive element reduces high frequency content of the voltage suppliedto the first coil whereby electromagnetic interference is also reduced.

In an embodiment the duration of the voltage fed through the inductiveelement is controllable to durations between zero and one quarter of aperiod corresponding to the predetermined highest resonance frequency.This is useful for controlling and varying the power supplied to thecapacitive load. This maximum duration is the first half of ahalf-period, where voltage builds up, and the second half of thehalf-period is then used for the voltage to decrease.

In an embodiment the resonance frequency is higher than the audiblefrequency range for humans. This ensures that sound caused byalternating between positive and negative half-periods of voltagesupplied to the first coil is inaudible.

In an embodiment the positive and negative half-period drivers eachcomprises an electronic switching element such as a solid statesemiconductor switch or a vacuum tube.

In an embodiment where an ozone generating device is connected to thesecond coil of the power supply apparatus to form an ozone-generatingapparatus, the apparatus can be operated according to a methodcomprising controlling the power supplied from the power supplyapparatus to the ozone generating device to a predetermined power level;supplying a flow of oxygen-containing gas to the ozone generatingdevice; and controlling the flow of oxygen-containing gas so as toobtain a predetermined concentration of ozone from the ozone generatingdevice.

In an embodiment the power supply apparatus includes a transformercomprising a core, a low voltage coil on the core, and a high voltagecoil on the core, where the high voltage coil has a plurality ofinsulating carrier substrates stacked in an overlaying arrangement, eachcarrier substrate carrying an electrically conductive trace with endportions, the trace forming one or more turns around the core, and aconnector pad connecting an end portion of the trace on one substrate toan end portion of a trace on an overlaying substrate.

Traditional transformers have two or more layers with several turns ineach layer where an outer layer is wound around an inner layer, andphysically adjacent turns in adjacent layers can be separatedelectrically by several turns. This requires very good insulationbetween layers in order to avoid arcing between layers. A high voltagetransformer according to the invention has the advantage that themaximum voltage between physically adjacent turns of the high voltagecoil is limited to the voltage difference between two electricallyadjacent turns. This minimizes the risk of arcing between turns, wherebya long lifetime of the coil can be expected. Further, the high voltagecoil of such a transformer can have a short length measured along thecore, whereby it can be made compact, and it can be manufactured with ahigh degree of precision compared to coils that are wound from a lengthof wire. The coil can be manufactured as one unit, and if needed theentire coil can easily be exchanged, and individual substrates carryingone or more turns can also be exchanged. Coils can be composed of asmany substrates as needed according to the actual application.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with regard to theaccompanying figures. The figures show one way of implementing thepresent invention and are not to be construed as being limiting to otherpossible embodiments falling within the scope of the attached claim set.

FIG. 1 shows schematically a first embodiment of a power supplyapparatus of the invention,

FIG. 2 illustrates the timing of the first and second half-perioddrivers in the embodiment in FIG. 1,

FIG. 3 is a cross section through a high voltage transformer used in theembodiment in FIG. 1, and

FIGS. 4 and 5 each shows a substrate carrying an electrically conductivetrace for use in the transformer in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 is shown a power supply apparatus 100 with a load 300 having aload impedance with a capacitive component C and possibly also aresistive component. The load 300 is therefore referred to as acapacitive load and is illustrated as a capacitor. The load 300 can beany capacitive load such as an ozone generating device. The power supplyapparatus 100 comprises a transformer 110 with a first coil 120 and asecond coil 130. The first coil 120 has a center tap 121, which isconnected to an inductive coil 150 and a switching element 151. Theswitching element 151 can be operated under the control of a controller160 to open and close and thereby establish and disestablish aconnection between the inductive coil 150 and a DC supply voltage.Switching elements 170 and 180 at respective ends of the first coil 120are also operated under the control of the controller 160 to establishand disestablish connections to ground. The switching elements 151, 170and 180 are preferably solid state semiconductor switching elements suchas CMOS transistors, SCR's or other fast switching elements. In someapplications it might be considered to use vacuum tube switchingelements. The second coil 130 of the transformer 110 has an impedancewith an inductive component L and possibly also a resistive component R.Thereby the complex impedance Z is of the form Z=R+jωL. The capacitiveload 300 is detachably connected to the second coil 130 of thetransformer 110 to form a resonance circuit with a resonance frequencyf_(r) determined by the capacitive component C of the capacitive loadand the inductive component L of the second coil 130 of the transformer110 in accordance with the formula f_(r)=½π√{square root over (LC)}. Thetransformer 110 also has a third coil 140 connected to the controller160.

In FIG. 2 is illustrated the operation of the power supply apparatus 100in FIG. 1. The resonance circuit formed by the capacitive load 300connected to the second coil 130 of the transformer 110 has a resonancefrequency with a corresponding period T. In a first half-period thecontroller 160 controls the switching element 151 and the switchingelement 170 to close, whereby electric current flows from the DC voltagesource through the inductive coil 150 and through the center tap 121into the upper half of the first coil 120 and through the switchingelement 170 to ground. The inductive coil 150 and the inductiveimpedance of the first coil 120 of the transformer 110 have the effectthat this current does not rise momentarily but exponentially towards anupper asymptote. After a period t the switching element 151 iscontrolled to open, and due to the inductive impedance in the circuitincluding the inductive coil 150 the current in the upper half of thefirst winding 120 continues but is now drawn through the diode 152rather than from the DC voltage source. The voltage over the switchingelement 180 decreases at a rate determined by the resonance frequency.After one half-cycle T/2 of the resonance frequency this voltage hasdecreased to zero the switching elements 170 and 180 are both controlledto change their state so that switching element 170 is opened andswitching element 180 is closed, and the next half-cycle begins.Electric current flows from the DC voltage source through the inductivecoil 150 and through the center tap 121 into the lower half of the firstwinding 120 and through the switching element 180 to ground. Afteranother period t the switching element 151 is controlled to open, andthe current in the lower half of the first winding 120 continues but isnow again drawn through the diode 152 rather than from the DC voltagesource. The voltage over the switching element 170 decreases at a ratedetermined by the resonance frequency. After another half-cycle, i.e.one full cycle, of the resonance frequency this process is repeated.

The actual resonance frequency determines the time when the voltage overthe open one of the switching elements 170 and 180 is zero, whichhappens after each half-period, which is when the switching of switchingelements 151, 170 and 180 is made. This time is determined using thethird coil 140 on the transformer. The coil 140 senses a voltage whichis in phase with the voltage over the open one of the switching elements170 and 180, which in particular means that zero crossings occursimultaneously. The voltages sensed by the third coil 140 is input tothe controller 160, and the controller 160 determines zero crossings ofthe voltage sensed by the third coil 140, at which times the switchingelements are controlled as described above.

The period t in which the switching element 151 is closed can be varied,and the switching element 151 may be controlled to open e.g. when thecurrent has reached a predetermined level. Hereby e.g. the average valueor the RMS value of the voltage on the first and second coils can becontrolled, and hereby the power delivered to the load can be varied.The maximum duration of the period t in which the switching element 151is closed is determined as no more than one quarter of a period Tcorresponding to a predetermined highest resonance frequency at whichthe apparatus is designed to operate.

In case of disconnection of the capacitive load during operation of theapparatus the resonance frequency will increase, which might causeundesired operating conditions, in particular if the switch 151 wereallowed to operate at such increased resonance frequencies. In order toavoid such conditions a maximum repetition frequency has been set forthe operation of the switch 151. This maximum repetition frequencycorresponds to the predetermined highest resonance frequency at whichthe apparatus is designed to operate or slightly higher.

In case of short circuiting of the terminals of the second coil 130during operation of the apparatus undesired operating conditions mightalso arise, in particular high currents in the first and second coils ofthe transformer. The opening of the switching element 151 when thecurrent has risen to a predetermined level limits the current that canbe drawn from the second coil, which is useful in case of shortcircuiting of the terminals of the second coil 130.

FIG. 3 shows an embodiment of a high voltage transformer 500 suitablefor use in the embodiment in FIG. 1. The transformer 500 has a core 501composed of two preferably identical E cores 502 and 503 with theirmiddle legs touching each other and thus in magnetic contact with eachother. Their outer legs are shorter than the middle legs whereby airgaps are formed in each of the outer legs of the core. A first coil 510is wound on a bobbin 511 and placed around the middle leg. A second,high voltage coil 520 comprising two half-coils with one half-coilplaced on either side of the first coil 510.

FIG. 4 shows an embodiment of the individual turns of the high voltagetransformer in FIG. 3. A flat sheet or substrate 600 of an electricallyinsulating material with a central opening 601 carries an electricallyconductive trace 610 forming a loop around the central opening 601. Atthe outer end portion 611 the electrically conductive trace 610 has aconnector pad 612 on the same side of the substrate 600 as theconductive trace 610, and at the inner end portion 613 the electricallyconductive trace 610 has a connector pad 614 on the opposite side of thesubstrate 600 with a through-going connection. The conductive trace 610can have one or more turns around the central opening 601.

FIG. 5 shows another embodiment of the individual turns of the highvoltage transformer in FIG. 3. A flat sheet or substrate 700 of anelectrically insulating material with a central opening 701 carries anelectrically conductive trace 710 forming a loop around the centralopening 701. The structure in FIG. 7 is a minor image of the structurein FIG. 6, except that at the outer end portion 711 the electricallyconductive trace 710 has a connector pad 712 on the opposite side of thesubstrate 700 with a through-going connection, and at the inner endportion 713 the electrically conductive trace 710 has a connector pad714 on the same side of the substrate 700 as the conductive trace 710.The conductive trace 710 can have one or more turns around the centralopening 701.

In FIG. 3 each of the half-coils of the high voltage coil 520 iscomposed by stacking alternating substrates 600 and 700. When asubstrate 600 is placed on top of a first substrate 700 in an overlayingarrangement, the pad 614 will be just above the pad 714, and the twopads 614 and 714 can be connected electrically, e.g. by soldering. Thethus interconnected traces 610 and 710 on their respective substrateswill thereby form two turns or loops around the central openings. Asecond substrate 700 can then be placed on top of the substrate 600 withthe pad 712 just above the pad 612, and the two pads 612 and 712 can beconnected electrically in the same manner to form a coil with threeturns. In this way several substrates 600 and 700 can be stackedalternatingly to form a coil with any desired number of turns. The highvoltage coil 520 of the transformer 500 comprises two half-coils whicheach are made like this. In FIG. 3 the high voltage coil 520 with itsthus stacked substrates is seen from the edge of the substrates.

The distance from the electrically conductive traces 610 and 710 to theedge of the substrate should be large enough to prevent arcing betweentraces on adjacent substrates.

As mentioned, in an embodiment the ozone generating apparatus describedabove will be operated at frequencies above the audible range forhumans, e.g. in the frequency range 15-25 kHz. This also has the effectthat the size of the transformer core can be reduced in comparison tothe size required at lower frequencies.

For high frequency purposes Litz wire is used for the first coil 510.Litz wire consists of a number of insulated wire strands which may betwisted or woven together. At high frequencies the electric current willflow in a surface layer of a thickness which decreases with increasingfrequency—this is the so-called skin effect. At 20 kHz the skin depth isabout 0.5 mm in copper. At the air gaps in the outer legs of thetransformer the stray magnetic field may influence the first coil 510.The use of Litz wire reduces the eddy currents in the first coil 510.

For high frequency purposes a laminated transformer core or a ferritecore can be used to reduce or eliminate eddy currents in the core.

The core 502, 503 has air gaps in the outer legs. Such a transformer isparticularly useful for to supply loads that exhibit negativeresistance, such as corona discharge devices used for ozone productionin an apparatus of the invention. At the air gaps there will be amagnetic stray field, and there is a distance from the first coil 510 tothe air gaps, and two half-coils of the second coil are kept apart sothat the windings are kept out of the stray field. At frequencies higherthan the audible frequency range for humans and power levels of severalkW as are handled in the apparatus of the invention the magnetic fieldwould dissipate considerable power in all metal parts subjected to thestray field, and it is therefore important to keep the stray field andall metallic components separate. This arrangement ensures that.

In some applications where ozone is used a predetermined yield of ozoneis needed or prescribed, or the required yield of ozone may change. Inan embodiment each of the flow of oxygen-containing gas and the powersupplied from the power supply apparatus to the ozone generating deviceis controlled so as to obtain a predetermined yield of ozone from theozone generating device and so as to minimize the consumption ofresources comprising oxygen-containing gas and power supplied from thepower supply apparatus. The control can be based on a mathematical modelof the apparatus and of the process including theoretical andexperimental data and may also include actual measurements of relevantparameters for use e.g. in a feedback control system.

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The scope of the present invention isset out by the accompanying claim set. In the context of the claims, theterms “comprising” or “comprises” do not exclude other possible elementsor steps. Also, the mentioning of references such as “a” or “an” etc.should not be construed as excluding a plurality. The use of referencesigns in the claims with respect to elements indicated in the figuresshall also not be construed as limiting the scope of the invention.Furthermore, individual features mentioned in different claims, maypossibly be advantageously combined, and the mentioning of thesefeatures in different claims does not exclude that a combination offeatures is not possible and advantageous.

What is claimed is:
 1. A method of operating an ozone generatingapparatus, the ozone generating apparatus comprising an ozone generatingdevice connected to a power supply apparatus for supplying electricpower from the power supply apparatus to the ozone generating device,the method comprising: supplying a flow of oxygen-containing gas to theozone generating device, controlling the flow of oxygen-containing gasand controlling the power supplied from the power supply apparatus tothe ozone generating device so as to obtain a predetermined yield ofozone from the ozone generating device and so as to minimize thecombined consumption of resources consisting of oxygen-containing gasand power supplied from the power supply apparatus.